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That happens from time to time. You probably just flagged shortly after a post was deleted, presumably while a moderator was working through the queue, or after several high reputation users cast delete votes.

I can't find the exact transcripts, but this same behavior has been mentioned in chat several times. Moderators sometimes see the reverse of this, we go to take action on something in the queue only to realize that it's no longer there. Sometimes, strange things happen when we cross the proverbial streams.

There are times when your flag will tip a threshold for Community to automatically remove something (typical for comments), but you don't receive a notification that the post had already been deleted when that happens.